In recent history, thin film coatings on substrates have been used for many purposes, including the production of thin film semicondutors. Apparatus and methods for the deposition of thin films of semiconductor and insulating materials for the fabrication of semiconductor devices generally involve high vacuum depositions. Conventional glow discharge depositions include the decomposition of at least one reactive gas to produce the thin films. The decomposition may be activated in one of many ways. The most common way includes the use of a pair of electrodes having a potential applied therebetween, either electrode facing opposite surfaces of the substrate. A reactive gas, such as silane, is activated by an rf frequency generator to initiate a glow discharge in the region between the electrodes, resulting in a deposition of a thin film of material on the surface of the substrate. In the silane example, various dopants may be introduced into the reactive gas to produce desired conductivity types of amorphous silicon thin films.
Commonly, amorphous silicon photovoltaic cells are produced by depositing sequential layers of P-type material, intrinsic material, and N-type materials. Other types of manufactured semiconductors utilize PN configurations and oxides, nitrides and other thin films. For the production of successful devices, it is imperative to minimize cross-contamination between each layer deposited. In certain deposition apparatus, the substrate is placed within the deposition housing, the atmosphere is evacuated to a pressure less than 10.sup.-7 torr, and the gas corresponding to the desired material is introduced into the deposition housing. A plasma is thereafter initiated, the deposition occurs, and the reactive gas is evacuated before the next gas is introduced. Residual gases not pumped out of the system before the next gas is introduced contaminate the material layer deposited thereafter.
One of the most plaguing problems in the production of semiconductors is contamination of the materials due to the presence of contaminants in the deposition chamber during the deposition process. These problematic contaminants come from various sources: (1) non-exhausted gases; (2) leaks around the seals and transportation mechanisms; (3) lubricants used within the deposition chamber; (4) heater elements, as well as other origins. Previous attempts to solve the contamination problem have included the use of a load lock chamber with a gate valve to separate the load lock from the deposition chamber to which it is connected. Furthering the problem, however, some systems include transportation mechanisms located outside the vacuum. In these outside systems, the transportation mechanisms reaching into the deposition chamber are likely to have leaks surrounding the point of entry. Lubricants outgas into the systems and dope the semiconductor layers, generally with carbon compounds.
Further, previous systems have included heater elements within the deposition chamber. Due to its location, the heater element may outgas and create contamination problems. To combat this outgasing problem, expensive heater elements which do not outgas have been utilized to prevent contamination of the material layers.
Another plaguing problem in the production of semiconductors is a lack of uniformity in the deposition of the semiconductor material layers themselves. Many different methods have been proposed to minimize non-uniformity problems including introducing the reactive gases used in the deposition system from various angles, the utilization of gas curtains and the like, and metering apparatus for the careful introduction of reactive gases at certain rates.
The configuration of the deposition chamber itself in previous systems has unfortunately been limited to gas-specific applications. It would be advantageous for a deposition system to exhibit a capacity for depositing many different types of gases to permit the deposition system to be used in many different ways. Prior art systems, although technically are available for use in the deposition of various coatings, are not adjustable to obtain optimum results when introducing new and different reactive gases. The anode-cathode separation distance is essentially gas-specific.
Transportation systems in prior art multi-layer coating mechanisms have included many different devices including axially rotated positioning means, small automated rail car systems, push rod mechanisms, and web transportation means. Many other transportation systems have been attempted. Some transportation systems are located outside the deposition chamber, inserting the substrate into the deposition chamber in each instance. All of the above-mentioned methods have problems that accompany each one. An efficient, clean, inexpensive transportation mechanism is desirable to solve these problems.